166 PROF. OSBORNE REYNOLDS ON 



remarkable than the experimental ; for while the experi- 

 ments only show that diminishing the pressure in the 

 receiving vessel below a certain limit does not increase 

 the flow, the equations show that by such diminution of 

 pressure the flow is actually reduced and eventually stopped 

 altogether. 



In one important respect, however, the equations agree 

 with the experiments. This is m the limit at which 

 diminution of pressure in the receiving vessel ceases to 

 increase the flow, which limit by the equations is reached 

 when the pressure in the receiving vessel is "527 of the 

 pressure in the discharging vessel. 



The equations referred to are based on the laws of 

 thermodynamics, or the laws of Boyle, Charles, and that 

 of the mechanical equivalence of heat. They were inves- 

 tigated by Thomson and Joule (see Proc. Roy. Soc, May 

 1856), and by Prof. Julius Weisbach (see ' Civilingenieur,^ 

 1856) ; they were given by Rankine (articles 637, 637 a. 

 Applied Mechanics), and have since been adopted in all 

 works on the theory of motion of fluids. 



Although discussed by the various writers, the theory 

 appears to have stood the discussion without having re- 

 vealed the cause of its failure; indeed, Hirn, in a late 

 work, has described the theory as mathematically satis- 

 factory. 



Having passed such an ordeal, it was certain that if 

 there were a fault, it would not be on the surface. But 

 that by diminishing the pressure on the receiving side of 

 the orifice the flow should be reduced and eventually 

 stopped, is a conclusion too contrary to common sense to 

 be allowed to pass when once it is realized ; even without 

 the direct experimental evidence in contradiction, and in 

 consequence of Mr. Wilde^'s experiments, the author was 

 lead to reexamine the theory. 



